This invention relates to instruments for measuring the distance from a hole to an adjacent flange, and more particularly to an instrument for measuring the distance from a hole, drilled at the intersection of a rib and a stringer, to the adjacent flange of the rib and the adjacent edge of the stringer flange to insure that the hole is drilled at the correct position so that a fastener, to be positioned in the hole for fastening the rib and stringer together, is correctly positioned.
In airplanes, as in many other mechanical systems, the fastening of two crossing L-shaped or Z-shaped structural elements is done by drilling a hole through the overlapping portions of the two elements, and securing them together with a fastener such as a rivet or a threaded fastener inserted in the hole. For optimum strength, the hole should be properly placed relative to the flanges of the structural elements to provide the maximum strength and appropriate clearance of the fastener head from the flange.
Quality control procedures to insure that the hole is properly located have included putty and steel scales which have poor accuracy, and "go, no-go" devices which indicate whether the holes are in tolerance, but give no indication how far in or out of tolerance the holes are. Also, transfer devices have been used which are satisfactory in some cases but are less accurate and more cumbersome and slower to use.
Modern quality control procedures include statistical process control which make it possible to track the position of the hole in a succession of parts, even when they are within tolerance, so that corrections can be made when it becomes apparent that the position of the hole is drifting toward an out of tolerance position. In this way, it is possible to initiate a correction before an actual out of tolerance condition occurs. However, in order for a statistical process control system to be possible, it is necessary to acquire accurate information as to the actual position of the hole in a succession of parts, as opposed to simple information as to whether the hole is within or out of tolerance. With those accurate hole locations, the statistical process control calculations and plots can be made and quality control corrections can be initiated to insure that expensive corrections for out of tolerance holes are unnecessary.